Sunlight weakness

Immortal though the vampires might be, they still had to sleep by day.
While their scientific prowess had spawned various antidotes for sunlight,
they never succeeded in conquering the hellish pain that came when their
bodies were exposed to it.

The agony of cells blazing one by one, flesh and
blood putrefying, every bodily system dissolving—even the masters of the
earth were still forced to submit to the legends of antiquity.

Though the vampires had reached the point where their bodies wouldn’t be
destroyed, many of the test subjects exposed to more than ten minutes of
direct sunlight were driven insane by the pain; those exposed for even five minutes were left crippled, their regenerative abilities destroyed. And, no matter what treatment they later received, they never recovered.

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